Here are a few reasons government forecasters at the National Hurricane Center and emergency management officials are so concerned about Sandy:

1. Sandy is one of the largest hurricanes ever to strike the U.S. Sandy's winds cover an area of more than 1,000 miles in diameter. That's enormous by hurricane standards. So instead of affecting an area a couple of hundred miles across, Sandy will cut a huge swath. That means many millions of people are probably going to be exposed to high winds, heavy rains, and, for those on the coast, powerful storm surge.

2. Sandy is a very slow-moving storm. Sandy was slowing as it turned inland in a northwesterly direction. That means many places could see two full days of heavy winds and rain, not just a few hours. Sandy is now packing winds of more than 90 miles per hour, according to latest update from the National Hurricane Center. Forecasters say some places will get a foot of rain and they expect widespread flooding, wind damage and power outages.

3. Sandy remains strong as it reaches the coast. Hurricanes often weaken as they travel north across colder water and approach land. But Sandy hasn't. One reason is that it's expected to change from a tropical storm powered by warm ocean water to something more like a winter storm powered by temperature and pressure differences in the atmosphere. So it will be a hybrid, or "Frankenstorm." And forecasters say Sandy may actually gain strength slightly as it reaches land. They also think it will remain strong enough once on land to produce strong winds far inland.

4. Sandy is arriving during high tides.October 29 is a full moon and the highest tides usually come a day or two after that. So the "life-threatening" storm surge caused by Sandy's winds is likely to coincide with several very high tides. Forecasters say in some places, the combined effect could push tidal waters 11 feet higher than normal — a level usually associated with much more powerful storms.

5. Sandy is bringing cold air and snow as well as wind and rain.Forecasters expect Sandy to meet a mass of very cold air from a winter storm about the time it reaches land. This cold air will be incorporated into the hurricane, which means in some places torrential rains will be followed by temperatures in the 20s. It's situation with "two events amplifying each other," David Nolan of the University of Miami told NPR's Joe Palca.

That's a big problem if you don't have power or heat. Also, Sandy is expected to produce two feet of snow or even more in parts of West Virginia. So the problems of extreme winter and summer weather will be wrapped into one storm.

6. Sandy is likely to affect New York City.Last year's Hurricane Irene was a reminder of how vulnerable Manhattan and the other boroughs are to storm surge. Even though Irene had weakened to a tropical storm by the time it reached Manhattan, it still caused the Hudson to flood the Meatpacking District. Many forecasters say Sandy could be much worse. Bad news in a place where many subway tunnels are lower than the Hudson already.

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Transcript

ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. Hurricane Sandy has earned the nickname Frankenstorm for a reason. An odd confluence of weather events have helped the storm grow and the result is a frightening mix: high winds, storm surge, snow and driving rain. But what is the science behind Sandy? A cold weather system mixing with a tropical system to become extratropical or post-tropical, depending on who you ask.

To translate, I'm joined by Perry Samson, professor of atmospheric science at the University of Michigan. Welcome, Mr. Samson.

PERRY SAMSON: Thank you.

CORNISH: So what makes this storm unique?

SAMSON: We have, as you've mentioned, a combination of events going on. First, the tropical storm, which is moving up the East Coast following the Gulf Stream. And tropical storms are driven by the availability of warm water and moisture. These cause the clouds, and these then clouds form into the storms that make a hurricane.

Now, that storm is following the Gulf Stream, as it would normally do, as it moves towards the north and east. But the odd thing that's happening now is that the upper level winds are grabbing that storm and pulling it back towards the coastline. And that's where the tropical storm then gets embodied into these - the jet stream and these waves in the upper atmosphere to produce an extratropical storm.

CORNISH: Explain the term extratropical.

SAMSON: These are the kinds of storms that we normally get in the United States, and they're produced by variations in the jet stream in the upper atmosphere. Without going into it, these variations cause the winds to rise in the atmosphere and cause low pressure. These are fairly well understood and studied by many.

The combination of this kind of storm with a hurricane embedded in it, I don't remember a storm like this in the last 30 or 40 years. It's going to produce tremendous amounts of precipitation across Pennsylvania and New York and New Jersey. In addition, this has the bad luck of coming onshore at high tide and with a full moon.

So we're concerned and interested in how this is going to affect storm surge, how the topography of the Long Island Sound and how the topography of New Jersey and New York might confound this. These are questions that are hard to answer.

CORNISH: I've also heard that Sandy has extremely low pressure. Could you explain what that means and what role pressure plays in a hurricane's strength?

SAMSON: Central pressure in Hurricane Sandy right now is somewhere around 950 millibars. And typically, the pressure at sea level is somewhere around 1,012 millibars. The pressure in the center doesn't by itself cause the storminess. It's the change of pressure between the outside the storm and inside the storm. This pressure gradient, the stronger it gets, the stronger the winds become. So it's the difference in pressure from inside to outside the storm.

CORNISH: Looking at this as an academic, are there any aspects of the storm or the way it's being described that you think are maybe hyperbolic, r, you know, people kind of getting overexcited?

SAMSON: I have a trouble with the word Frankenstorm myself, but that's just me. But beyond that, this particular storm is going to offer a huge opportunity for our science to study what happens when a tropical storm and a extratropical storm come together, because these are very rare events. And the fact that this is happening and hitting an area of high population gives great concern about the current situation but also a great opportunity in the future to know more about the dynamics of our atmosphere.

CORNISH: Perry Samson, thank you so much for talking with us.

SAMSON: It's been my pleasure.

CORNISH: Perry Samson is a professor of atmospheric science at the University of Michigan. Transcript provided by NPR, Copyright NPR.